Those things are a real pain. Some angled, partially covered by brackets, rusted in place, etc.
Agree with breaking them. Use a massive breaker bar (not some wimpy 3/8"-drive 12" bar) and snap them off. If you get them partially loose, they will simply wallow out the square hole in the bumper, then you have nothing keeping them from rotating.
I always put mine back together with a liberal coating of grease on the threads.